Should there be an additional cost during the Invisalign refinement process for taking the moldings of my teeth?

After sometime with Invisalign, I now need a refinement. My doctor wants to charge me to take new moldings of my teeth, but since the refinement itself is free, I was under the assumption this would be free as well. Is this additional charge normal in this process, or is this something I should be questioning? Thanks for the help!